Duration: Annual
Nativity: Native
Lifeform: Forb/Herb
General: Annual herb, 15-20 cm tall; stems erect, branching, the branches spreading to ascending; herbage minutely appressed-pubescent.
Leaves: Opposite along the stems and branches, with most leaves clustered near branch tips with the flowers; blade linear to lanceolate, 2-4 cm long and 3-30 mm wide, finely serrate, the tip pointed, and the base symmetric and tapered into a short petiole; with tiny glandlike stipules at the base of the petioles.
Flowers: Has the highly modified flower structure characteristic of Euphorbias. Structures called cyathia appear to be individual flowers, but are composed of fused-together bracts forming a cup (involucre), with peripheral nectary glands which are often subtended by petal-like bracts called petaloid appendages. Within the cup there is a ring of inconspicuous male flowers, each reduced to a single stamen. Out of the middle protrudes a single, stalked female flower which lacks petals. In Euphorbia exstipulata, the cyathia (modified flowers) are clustered into leafy cyme-like inflorescences at branch tips. Involucres are top-shaped or bell-shaped, and 1.5 mm high, and glabrous or hairy, with 4 pink to yellow glands around the edge, each with a tiny white to pink petaloid appendage which is sometimes divided into triangular lobes; 10-12 staminate flowers.
Fruits: Capsule distinctly 3-lobed, depressed-oblong to ovoid, 3 mm wide, sparsely hairy; seeds white to gray or light brown, ovoid, 2 mm long, bluntly 4-angled in cross section, tuberculate (bumpy), often with 2 transverse ridges.
Ecology: Found in dry rocky spots from 3,000-6,500 ft (914-1981 m); flowers August-October.
Distribution: s CA, s UT, AZ, NM, s TX; south to c MEX.
Notes: Distinguished by being a low, erect annual with opposite branching on the lower stems and often forked branching in the inflorescence; narrow opposite leaves, most of them with serrate edges; leaves below the flowers all green (in other species these can be bicolored); flowers have toothed white petaloid appendages; and angled seeds. Broad-leaved plants are sometimes segregated as var. lata (not to be confused with Euphorbia lata, a synonym of Chamaesyce lata and a completely different species), but the variation in leaf shape is continuous and FNA and Allred do not formally recognize any varieties. E. davidii is similar but is generally a larger plant, has only one gland per involucre, and lacks petaloid appendages. This species is a member of the Pointsettia section of Euphorbia.
Ethnobotany: Unknown, but other species in the genus have uses.
Etymology: Euphorbia is named for Euphorbus, Greek physician of Juba II, King of Mauretania; exstipulata means lacking stipules, alluding to the tiny reduced gland-like stipules.
Editor: SBuckley 2010, FSCoburn 2015, AHazelton 2017
